The invention relates to a drive-slip control device (ASR) for a braking system on a road vehicle also equipped with an anti-lock system (ABS). The braking system includes a dual circuit static brake system, in which one brake circuit is assigned to the driven vehicle wheels and the other brake circuit to the non-driven vehicle wheels and wherein the two brake circuits are assigned to an outlet-pressure space of a brake unit provided for brake actuation. The system operates according to the following features:
(a) the ABS operates (at least on the brake circuit of the driven vehicle wheels) on the return-flow principle whereby, during pressure-reduction phases of the anti-lock control, a quantity of brake fluid (corresponding to that quantity of brake fluid caused to flow off from the wheel brake(s) into a return line connected to a buffer accumulator during ABS control) is pumped back into the brake unit by a return pump having an inlet side connected to a buffer accumulator and an outlet side connected to the brake unit;
(b) the ASR operates on the principle of decelerating a vehicle wheel tending to spin by subjecting its wheel brake to brake pressure from an auxiliary-pressure source, in such a way that its drive slip remains within a value range compatible with a sufficient dynamic stability of the vehicle;
(c) wherein during ASR control, brake fluid is conveyed from a reservoir of the brake unit to an inlet side of the return pump by a precharging pump (connected as an auxiliary-pressure source to the inlet side of the return pump) via an outlet non-return valve and from which (in pressure build-up phases) pressure is supplied to the wheel brake of the vehicle wheel to be decelerated by means of the control;
(d) wherein there is an ASR control valve which can be changed over from a normal braking basic position, in which brake fluid can be positively displaced into the wheel-brake cylinders of the brakes of the driven vehicle wheels as a result of the actuation of the brake unit, into an alternative functional ASR position, in which brake fluid is prevented from flowing off from the main brake line of the driving wheel brake circuit towards a respective outlet-pressure space of the brake unit; and
(e) wherein there is a valve arrangement which is activatable in response to the outlet pressure of the precharging pump.
An ASR of this general type is the subject of the applicant's older, not previously published German Offenlegungsschrift No. 3,802,133 which describes, in combination with an ABS working on the known return-flow principle, an ASR working on the principle of decelerating a driven vehicle wheel tending to spin by subjecting its wheel brake to pressure. There the return pump of the ABS, assigned to the brake circuit of the driven vehicle wheels, is utilized as an auxiliary-pressure source for the ASR. This return pump is designed as a free-piston pump and is fed brake fluid from the brake-fluid reservoir of the brake system into the return line of the ABS by a precharging pump via its outlet non-return valve (in the ASR control mode). For generating brake pressure in the ASR mode, the return pump conveys brake fluid into the main brake line which is shut off from the brake unit by means of an ASR control valve. Furthermore, there is an ASR outlet valve which is connected between the return line of the brake circuit of the driven vehicle wheels and the brake-fluid reservoir. The basic position of this ASR outlet valve is its shutoff position, and only in pressure-reduction phases of the ASR is it moved into its throughflow position in which brake fluid can flow from the return line towards the brake-fluid reservoir.
During normal braking (braking not undergoing a brake-pressure control), this return line is pressureless. Consequently, a leak (in the admission-pressure supply path of the return pump in the return line and utilized for the drive-slip control), cannot be detected during normal braking. Such a leak could occur at the outlet non-return valve of the precharging pump or at the ASR outlet valve.
However, if such a leak is present and the front-axle brake circuit fails during a braking operation, then, during this braking operation, the ABS on the rear-axle driven vehicle wheel brake circuit allows this brake circuit to empty via the leak. Thus both brake circuits will fail. This complete failure of the brake system is permanent. The potential danger that with a failure of the front-axle brake circuit, the rear-axle brake circuit will also fail if the latter has a leak in the return-pump supply circuit, is naturally unacceptable. This is especially true since the probability of failure of the rear-axle brake circuit is relatively high because, by virtue of construction, it opens towards the brake-fluid reservoir via the ASR outlet valve and because the precharging pump is connected to the return line.
The object of the invention is, therefore, to improve on an ASR of the above-mentioned type, to the effect that a leak in the admission-pressure supply circuit of the return pump becomes clearly detectable (by a pedal reaction characteristic) during the actuation of the brake system even during normal braking. This danger of a complete failure of the brake system can be countered in good time.
According to the invention, this object is achieved by having a valving arrangement that is responsive to the outlet pressure of the precharging pump connect both the outlet non-return valve of the precharging pump and the ASR outlet valve (connected between the brake-fluid reservoir and the return line) to a pressure-medium line (subjected to brake pressure in a braking mode and in the drive-slip control mode), and also connect both a pressure-medium line (connecting the ASR outlet valve to the return line) and the outlet of the precharging pump to the inlet of the return pump of the brake circuit of the driven vehicle wheels.
By means of such a valving arrangement (one activatable by means of the outlet pressure of the precharging pump and in the braking mode connects the ASR non-return valve of the precharging pump and the ASR outlet valve to a pressure-medium line, subjected to the brake pressure and in the drive-slip control mode connects this pressure-medium line to the return line of the brake system and the outlet of the precharging pump to the inlet of the return pump, which acts as an ASR auxiliary-pressure source) during normal braking both the ASR outlet valve and the outlet non-return valve of the precharging pump can be checked in terms of leaks. A leak of these functional elements is detectable from a disproportionate yielding of the brake pedal.
A fault detection of this kind can be demonstrated objectively utilizing a brake-pressure sensor and a displacement sensor for monitoring the position of a brake-unit piston. Output signals between these two must be in a specific ratio to one another (assuming proper functioning of the brake system) and can be determined by a comparison of the two sensor output signals. If this ratio differs from a predetermined value (characteristic of the proper functioning of the brake system), this can be signalled to the driver acoustically and/or optically. On the assumption that the brake unit is designed as a tandem master cylinder, it is also possible to use the output signals from two displacement (or position) sensors which monitor the positions of the pistons movably limiting the outlet-pressures spaces of the master cylinder. The position of the pistons during the proper functioning of the brake system should be the same and thus the sensors should generate output signals in a specific ratio to one another.
By having the ASR control valve moved by the outlet pressure of the precharging pump into its functional position shutting off the brake unit from the main brake line of the brake circuit of the driven vehicle wheels, reduces the outlay in terms of electronic and electromechanical control and also increases functional reliability.
The same results are also obtained when the ASR outlet valve is moved by the outlet pressure of the precharging pump into its functional position making the connection between the return line and the brake-fluid reservoir and where there is a first non-return valve which is connected between the return-pump inlet and the return line of the brake circuit of the driven vehicle wheels. Here this first non-return valve is shut off by means of a relatively higher pressure at the pump inlet than the pressure in the return line. A second non-return valve is connected between a common connection of two of the hydraulically controlled outlet valve arrangements and the first outlet non-return valve. However, in this case it is necessary to have the two additional non-return valves to shut off the outlet of the precharging pump from the brake-fluid reservoir of the brake system.
By having a supply control valve, which is controlled by the outlet pressure of the precharging pump in the functional position in which it connects the delivery outlet of the precharging pump to the inlet of the return pump of the brake circuit of the driven vehicle wheels via the outlet non-return valve, also achieves a simple and functionally reliable design of the hydraulic unit of the ASR.
If there is a control slide having an actuating piston which is activatable by means of the outlet pressure of the precharging pump to cause controlled displacement of mechanical control elements that change-over the position of the jointly switchable valves, then it is advantageous if there is a control-pressure reducing valve which, during an actuation of the brake system, connects the control line providing control pressure for the hydraulically activatable valves to the brake-fluid reservoir. This allows for a substantially delay-free transition from a drive-slip control to the normal braking mode, or a braking mode which can undergo anti-lock control.
It is desirable if the control-pressure reducing valve can be moved into its functional position connecting the control line to the brake-fluid reservoir as a result of the displacement of one of the brake-unit pistons occurring during an actuation of the brake unit, or if the control-pressure reducing valve is designed as a change-over valve controlled by brake pressure.
If the structure of the precharging pump is that of a piston pump which can be driven by means of the drive provided for the return pumps, then an electrical drive motor normally provided for the precharging pump is not necessary.
A necessary design of the precharging pump as a self-priming pump can be achieved by having the precharging pump equipped with a displacement-controlled inlet valve which is moved into its open position while the pump piston is executing the final portion of its suction stroke and the initial portion of its delivery stroke and which otherwise remains in its shut off position.
A non-return valve which is moved into its throughflow position by a relatively higher pressure at the pump outlet than the control and into its shut off position by relatively higher pressure in the control line than the pump outlet, simply ensures that the valves to be changed over at the onset of the drive-slip control and which are controlled by means of the outlet pressure of the precharging pump, remain held in their functional positions for the duration necessary for a drive-slip control cycle.
A brake monitoring device (DE 3,443,880 Al) for a vehicle brake system which is provided with an anti-lock system and operates with a hydraulic accumulator, the outlet pressure of which is monitored is known. In contrast thereto, in order to check the ability of the brake system to function, two pressure switches, which respond to different pressure thresholds in the brake system, can be utilized to detect the functioning of the brake system in a simple manner.
A fault detection device which generates an indicator signal characteristic of the proper or faulty functioning of the brake system can be obtained as a result of the processing of a first sensor output signal, which is a measure of the displacement of a piston of the brake unit occurring as a result of the actuation of the brake system, along with a second sensor output signal, which is a measure of the brake-actuating force arising as a result of the actuation. The second sensor output signal is generated by a pressure sensor which monitors the brake pressure occurring in one of the brake circuits of the vehicle (preferably the brake circuit of the driven vehicle wheels).
If the brake unit is being designed as a tandem master cylinder, then the fault detection device can comprise two displacement sensors each monitoring one of the two pistons of the tandem master cylinder and generating electrical output signals characteristic of the piston positions. As a result of a comparative processing of these output signals, the fault detection device generates indicator or warning signals characteristic of the proper or faulty functioning of the brake system.
The ASR control valve can alternatively be connected between the brake unit and the brake-fluid reservoir.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.